A lithium ion secondary battery is known as an advantageous secondary battery which is able to give a high voltage of about 4 volts and a high discharge capacity. As the positive electrode active material of the lithium ion secondary battery, LiMn.sub.2 O.sub.4 having the spinel crystal structure, as well as LiMnO.sub.2, LiCoO.sub.2, LiCo.sub.1-x Ni.sub.x O.sub.2 or LiNiO.sub.2 which has the rock salt crystal structure have been generally employed. The LiCoO.sub.2 having the rock salt crystal structure shows higher voltage and higher discharge capacity than other oxides and therefore is advantageous. However, the LiCoO.sub.2 has such drawbacks that cobalt is high in its cost and less available than other metals, and moreover may cause environmental pollution if battery wastes containing the lithium cobalt oxide are left outside
Japanese Patent Provisional Publication H3(1991)-147276 proposes a lithium ion secondary battery using LiMn.sub.2 O.sub.4 of the spinel crystal structure as the material for its positive electrode, that is, cathode. Manganese is less expensive and easily available, and moreover scarcely causes environmental pollution. However, LiMn.sub.2 O.sub.4 gives a charge capacity (corresponding to amount of releasable lithium ions) per unit volume less than LiCoO.sub.2 by 10 to 20%. This means that if the LiMn.sub.2 O.sub.4 is combined with a negative electrode active material of high capacity to prepare a secondary battery, the volume of the LiMn.sub.2 O.sub.4 (namely, positive electrode active material) to be used should be increased so as to balance its capacity with the high capacity of the negative electrode active material. As a result, the amout of the negative electrode active material encased in a container of a battery should be reduced, and then the battery capacity lowers.
Japanese Patent Provisional Publication H4(1992)-147573 describes a lithium ion secondary battery using Li.sub.1+M Mn.sub.2 O.sub.4 (x&gt;0) as the positive electrode active material precursor in combination of a negative electrode active material precursor such as carbonaceous material. Such positive electrode active material precursor--negative electrode active material precursor combination in a container of a battery is electrochemically converted into a positive electrode active material--negative electrode active material combination by electrically charging thus prepared battery so as to release lithium ions from the positive electrode active material precursor and intercalate the released lithium ions into the negative electrode active material precursor in the container.
The Li.sub.1+x Mn.sub.2 O.sub.4 having a lithium ion amount higher than LiMn.sub.2 O.sub.4 is advantageous because it gives a charge capacity higher than LiMn.sub.2 O.sub.4. In the lithium ion secondary battery described in the last Publication, the positive electrode active material of Li.sub.1+x Mn.sub.2 O.sub.4 is produced electrochemically once in an electric cell or chemically in a known process outside an electric cell and then placed in a container of a final battery product. Li.sub.1+x Mn.sub.2 O.sub.4 is known to be extremely unstable and easily oxidized. Therefore, this process and the secondary battery prepared by this process may cause problems in industrial preparation and use.